The invention concerns an electrical heating device for glass plates, especially mirror glass, by means of a multisectional, preferably metallic electrically conductive heating lining, which serves for heat distribution, whereby between the overlapping sections of the heating lining, which are insulated from each other and provided with electrical contacts, there are provided PTC resistor platelettes which come into direct contact with the sections on the top and on the bottom.
A heating device of this kind has become known from the German Pat. No. 29 19 968, which describes a heating device for outside mirrors of motor vehicles, which is provided with a bipartite configured metal lining made of aluminum sheet. Both parts of the lining partially overlap, whereby the platelette-shaped PTC resistors are disposed in the overlapping part of the lining. Current is supplied via the two tin plates of which each is provided with a tip-stretched electrical flat plug. This configuration provides a conductive adhesive as a mechanical connection between the heating resistors and the two tin plates. Furthermore, the two tin plates are adhered next to each other on the backside of a mirror by interposing a heating resistor. This kind of installation, however, has the disadvantage, that the actual heating cannot be accomplished independently, as is required in the production or during shipment, i.e. before the adhering on the mirror. In addition to the resulting low mechanical cohesion of the heating configuration, there is the further essential disadvantage, that on the rear of the mirror lining, potential differences are applied, which cause equalizing currents in the mirror lining, and thereby cause the mirror to become gradually blind. This danger is all the greater, because nowadays, mirrors of this kind no longer are provided with a lacquer coating as before, so that the chrome layer, which constitutes the actual mirror lining, is separated only by the adhesive film from the metal foil on which are applied potential differences.
In order to avoid this difficulty, it has already been proposed in German Utility Model No. 81 25 567 to provide a single large area metal foil, and to form the second overlapping metal lining by a current bridge which, separated by an insulation layer, is disposed on the backside of the large area metal lining. This configuration, however, requires constructionally complicated, and operationally not very satisfactorily mounting and contact brackets in order to reach the electrical contacts of the PTC platelettes. This configuration results in a very complicated construction, especially if a rivet joint is provided between the electrical connection brackets and the metal lining.
The German Utility Model No. 82 21 422 does provide a simpler mechanical construction of a mirror heating device by providing notches in the heating lining in which an insulated bracket can be positioned which makes contact with the PTC platelettes positioned directly on the heating lining. In this variant, however, there again is the main disadvantage of occurring potential differences on the backside of the mirror which, based on galvanic processes due to the equalizing currents, causes the mirror gradually to become blind.
The object of the invention is to provide an electrical heating device for glass plates, especially of mirrors, which is simple to produce, sturdy in use, which fits flexibly into unevenness and on curvatures of the glass plate surfaces, and which finally quantitatively avoids the difficulties of equalizing currents, i.e. the danger of causing the mirror to become blind.
For attaining the objective, in the initially mentioned embodiment of the heating device, it is provided that over the first heating lining which has to be heated, there is an insulation layer with openings for the PTC platelettes. On the insulation layer, there is disposed a large area second heating lining, which essentially overlaps the first heating lining.
Extremely simple production coupled with a flexible, mechanically very strong construction is achieved by avoiding great connecting bridges for making contact with the bottom or the top of the PTC platelettes, and by providing two, essentially equally large metal linings, between which the PTC platelettes can be arranged furthermore in random number and configuration.
Production can, for instance, very simply be achieved, whereby an insulation film provided with a double-sided adhesive coating is taped as an insulation layer on the backside of the first heating lining, whereby the PTC platelettes provided with a double-sided adhesive coating are pressed into the openings of this insulation film, and that on top thereof subsequently is positioned the second large area metal lining in the form of a copper, aluminum foil or the like. By providing an opening on the corner of the upper heating lining, there results the simple possibility of soldering the electrical connection wires to the heating linings. The large area adhesive bonding of both metal linings means that the electrical contact points between the frontal area of the PTC platelettes and the heating linings do not have to assume any mounting function, because the entire mechanical mounting is achieved by the large area adhesive bond. Accordingly, in the configuration of the invention, contact adhesive also can be utilized without problems between the PTC platelettes and the heating linings, without thereby encountering the difficulties that have been described in connection with German Pat. No. 29 19 968. The upper heating lining which has to be connected with the second pole of the source of current, comes nowhere into contact with the surface of the glass that has to be heated. Therefore by utilization of a heating configuration for heating mirrors according to the invention, compensating currents cannot occur, and thereby the danger of mirrors going blind is quantitatively avoided. All that is needed for patching a desired number and distribution of PTC resistors is solely the provision of corresponding punched openings in the insulation film, which can be achieved without substantial costs. In comparison thereof, in a configuration according to German Pat. No. 29 19 968, an even greater separation of the lining into unit sections would have to be effected, which finally would only cause that such a multisectional lining held together only via the PTC platelettes would become unmanageable. In a configuration according to German Utility Model No. 81 25 567, purely mechanically speaking, an extremely complicated construction would result, and even then, it would not constitute a random distribution of PTC platelettes on the heating lining, which would correspondingly be able to affect the defrosting image of a mirror.
In a configuration according to German Utility Model No. 82 21 422, there also can only be realized certain distributions of the PTC in a mechanically acceptable construction whereby, however, there still remains the essential difficulty of the thereby extensively occurring equalizing currents in the mirror lining.
The above discusses a very simple construction possibility of a heating device for glass plates, especially mirrors. According to the invention, the possibility of utilizing contact adhesive as an electrical conductive connection of the PTC platelettes with the heating lining has considerable construction technical advantages in comparison to soldering. And finally this permits also the special construction with two large area heating linings which, in addition to the already mentioned mechanical advantages, also results in an optimal heat distribution by the greatest possible avoidance of the so-called "dog-bone-effect" in the thawing image of the mirror. Furthermore, the missing soldering also has the advantage that the connecting wires can even subsequently be soldered to the heating linings, and that thereby no complicated flat plug configurations have to be utilized.
Especially in batter-supplied heating of motor vehicle outside mirrors, during which only relatively low voltage occurs, so that therefore also the insulation film between both heating linings can be configured relatively thin, it is advantageous to provide the upper heating lining with indentations for the PTC platelettes. By means of these indentations, the varying thickness of the PTC platelettes is compensated respective to the insulation material film. It furthermore causes a special embedding and thereby mechanical holding.
Furthermore it would be possible, in a configuration preferably especially for heating devices powered by line voltage, for example for aquariums or bathroom mirrors, to choose the thickness of the insulation layer so that it corresponds to the thickness of the PTC platelettes. In this case, both heating linings can be configured as entirely flat metal plates. The thick insulation layer, which again preferably consists of a correspondingly thick plastic film, which is provided with a double-sided adhesive coating, provides sufficient insulation between the heating linings to which are applied the potential differences.
In a further configuration of the invention, it can especially advantageously be provided that the heating device is embedded between two thermally good conductive insulation material films, i.e. that it is welded in. In this way is obtained a mat-shaped heating device which is completely insulated on all sides, which especially in operation with line voltage, as is advantageous for very large bathroom mirrors, allows not only an extremely simple installation behind the mirror, but also does not require a safety insulation transformer, because even if the mirror falls down and shatters, the danger of exposing current charged metal parts does not exist.